Offset Iterator

The offset iterator provides a convenient means to specify telescope sequences e.g.mosaics, dithers, nods. The main features of the offset iterator are identified in the figure below:

The coordinate system for offsets is defined as (p,q) where q is always parallel to the detector columns (or spectrograph slit) and p to detector rows (or orthogonal to the slit). The sketch in the lower left corner of the offset iterator is a useful reminder. Offsets are always entered in arcsec relative to the base position. For example, using a long-slit spectrograph like GMOS or GNIRS, an offset of (0,q) where q is any real number is always along the slit regardless of the position angle on the sky.

This (p,q) coordinate frame, and the offset positions, rotates with the image rotator. Thus only one mosaic is required regardless of the specified position angle. Similarly, if the base position is moved (i.e. translated) the offsets will move with it. With an offset pattern displayed in the position editor, you can drag the instrument rotation angle or base position to see this behavior.

The concept behind the telescope offset iterator is similar to the instrument iterators. You build up a sequence of telescope offset positions in an iteration table.The table columns are (a) Index, (b) p offsets, (c) q offsets and (d) the guide state (on/off). Table rows correspond to iterator steps. At run time, all the values in a row are set at once.

As many offset positions as desired may be entered, and the ordering of the list (the execution order) can subsequently be modified using the up and down arrow buttons. The selected offset, highlighted in green, can be moved up or down one step using the yellow arrows, or moved to the top or bottom of the list using the arrows with a bar. You can select and display (or re-order) multiple steps at once. To select several adjacent steps, click on the first one in the iteration table then shift+click on the last one; all steps in between will be selected (highlighted). To select non-contiguous steps ("disjoint sections"), select the first step and then ctrl+click on any others that you wish to select.

Steps in the list can be removed individually with the single red X button, en masse with the red XX button, or new steps may be added with the green + button. The values of a particular (highlighted) offset can be changed using the p and q editing boxes.Often it will be more convenient to change offset coordinates interactively in the position editor by dragging any offset as the science fields of view corresponding to the offsets and WFS probe vignetting are also displayed.

The "Grid..." button in the lower right of the offset iterator allows rapid creation of rectangular grid patterns. Set the offset of the initial position, the spacing and grid dimensions, then select create. Subsequent grids (e.g. to map an area of nearby sky) can be appended to the offset list. The order of steps in the list can be modified as described above to create arbitrarily complex patterns.

The "Random..." button can be used to quickly create sequences of random offsets that populate a square of the specified maximum offset in p,q.

If there are steps in the offset table where the guiding is listed as "off" this means that the WFS will not be moved from its previous position (i.e. it will not compensate for the telescope offset and will not attempt to stay locked on the star). This is useful if you wish to maintain the same illumination (vignetting) pattern on the detector e.g. for sky frames.

From the offset iterator it is possible to step through the sequence and display the science area and WFS probe positions in a (rudimentary) "playback" mode:

Configure the Position Editor to display the science FOV only at the selected position:
View > Science Display > Display Science FOV at > Selected Position

Configure the Position Editor to display Base, Offset, Science and the WFS FOV

Select (highlight) the first step in the offset list

Step through the offsets using the down-arrow key on your keyboard

The position editor viewer will update at each step

The "Advanced Guiding Options" tab is used to define the guide state of each guider individually, and should not be necessary in normal use.

The Save button accepts the latest changes and stores the program to the local database, and the Close button closes the science program editor (saving any changes to the local database).